Over the years, numerous efforts have been devoted to the improvement of techniques for the production of oxygen and nitrogen by cryogenic distillation to lower production costs, which consist mainly of power consumption and equipment costs. As a general rule, an efficient process usually requires an increase in equipment cost such that the overall gain is a result of a trade-off between power and capital costs. Therefore there is a constant need to come up with an efficient and low cost process to assure a significant reduction in the final product cost.
The invention described below utilizes the concept of high pressure distillation to reduce the equipment cost of cryogenic equipment. Also, by incorporating a power recovery scheme, the separation power for oxygen and nitrogen can be improved. The net result is a reduction of equipment cost and power cost leading to a reduction in the production cost of oxygen and nitrogen.
Traditionally, most air separation units are designed for relatively low air pressure (about 5-6 bar absolute) in order to minimize the power consumption of the air compressor which is the significant portion of the overall power consumption. Oxygen or nitrogen products can be compressed to higher pressure to suit. Product compressors or an internal compression process with liquid pumped feature can be used. This low pressure process results in several penalties for equipment cost namely: large piping and equipment size (exchangers, columns) due to pressure drop constraints at low pressure, and large and complicated (high number of stages) product compressors due to the availability at low pressure of oxygen and nitrogen product. The reduction of power consumption therefore rapidly approaches an asymptotic value dictated by the prohibitive cost of the equipment. Not only does this low pressure process penalize the cryogenic equipment, it also has a negative impact on the warm end equipment as well. Indeed, cryogenic processes require the feed gases to be free of impurities, such as moisture and CO.sub.2, which can freeze and plug the equipment at low temperature. Molecular sieve adsorption vessels with feed gas pre-cooling are used to remove these impurities. The lower the feed air pressure, the more difficult the adsorption process and the more adsorbent will be needed for the removal of impurities. Larger vessels and piping will also be needed to accommodate the low pressure drop. Overall, there is significant increase in equipment cost associated with the power cost reduction of the low pressure process.
Most of the negative effects caused by the low pressure can be eliminated if a high or elevated pressure process is used. A high pressure process is characterized by a high operating pressure in the low pressure column of a double-column process. By raising the pressure of the low pressure column from about 1.5 bar of the low pressure process to an elevated pressure as high as 2 to 7 bar, the feed air pressure needed for the high pressure column must be raised to as high as 20 bar. This high pressure results in very compact equipment for both warm end and cryogenic portions of the plant and significant cost reduction can be achieved. However, the high pressure process is detrimental and not favorable for a distillation operation, especially for the classical double column process. Indeed, when the low pressure column is operated above 3 bar absolute we can expect important loss of product recovery due to inefficient distillation and therefore high power consumption is unavoidable. Furthermore, the high pressure process will yield the nitrogen and oxygen products at elevated pressure and if only oxygen is needed as final product then the energy contained in the pressurized nitrogen must be recovered; otherwise inefficiency of the process will occur.
Several high pressure processes in cryogenics for air separation are described in the following patents.
U.S. Pat. No. 4,224,045 describes a high pressure plant where the feed air for an air separation unit is extracted from a gas turbine. The nitrogen product is recompressed for re-injection into the gas turbine loop for power recovery.
U.S. Pat. No. 4,947,649 describes a high pressure plant using a single column with nitrogen recycle heat pump to perform the air separation, instead of a double-column process. The feed air can be extracted from a gas turbine and the nitrogen product can be re-injected back into the gas turbine circuit.
U.S. Pat. No. 5,081,845 describes an integrated cryogenic air separation unit power cycle system wherein the air separation unit (ASU) is operated at elevated pressure to produce moderate pressure nitrogen. The integrated cycle combines a gasification section wherein a carbon source, e.g. coal, is converted to fuel and combusted in a combustion zone. The combustion gases are supplemented with nitrogen from the ASU and expanded in a turbine. Air to the cryogenic ASU is supplied via a compressor independent of the compressor used to supply air to the combustion zone used for combusting the fuel gas generated in the gasifier system.
U.S. Pat. No. 5,635,541 describes the possibility of using a high pressure process for oxygen production in remote areas where the power/fuel cost is low. A pressurized nitrogen product is expanded either across a valve or a power recovery turbine. This process emphasizes the cost reduction over the efficiency improvement.
U.S. Pat. No. 5,231,837 describes a triple-column process for high pressure application wherein a liquid rich in oxygen (rich liquid) of a high pressure column is further treated in an intermediate column to yield additional liquid reflux for a low pressure column. The intermediate column is reboiled by condensing nitrogen from the top of the high pressure column. A portion of the bottom liquid of the intermediate column is then vaporized in the overhead condenser of this column to yield a vapor feed to the low pressure column. By using this arrangement, the distillation process of the low pressure column is greatly improved, resulting in good oxygen recovery. If the air pressure or the low pressure column pressure are not too high, one can extract a significant amount of nitrogen product from the high pressure column to further improve the power consumption.
U.S. Pat. No. 2,699,046 describes processes wherein a rich liquid of a high pressure column is treated in a column or combination of columns reboiled by condensing the gases extracted from an intermediate level or from several levels of the high pressure column.
Several other high pressure or triple-column processes (often known as Etienne column processes) are also described in following patents published applications: U.S. Pat. Nos. 5,257,504, 5,438,835, 5,341,646, EP 636845A1, EP 684438A1, U.S. Pat. Nos. 5,513,497, 5,692,395, 5,682,764, 5,678,426, 5,666,823, and 5,675,977.